Japanese Patent Application Laid-Open Publication No. 2001-230774 (Patent Document 1) and Japanese Patent Application Laid-Open Publication No. 2004-253975 (Patent Document 2) disclose a method for achieving the multicast communication using unicast packets. Concretely, a packet conversion device on a transmission side converts a multicast packet from a server into a unicast packet and then transmits it to a packet conversion device on a reception side. The packet conversion device on a reception side converts the unicast packet into a multicast packet and then transmits it to a client.
Here, the packet conversion device of the Patent Document 1 appropriately rewrites destination/source IP addresses of a packet and a UDP port number at the time of conversion between the unicast packet and the multicast packet. In order to perform this rewriting, each packet conversion device is configured to retain a variety of information (IP address and UDP port number) before and after the conversion through mutual communications together with predetermined identifiers. Concretely, when a request from a client is received, the packet conversion device on a reception side starts communication to the packet conversion device on a transmission side on the assumption that the packet conversion device on a reception side knows the IP address of the packet conversion device on a transmission side in advance, and each packet conversion device determines the above-described retained information during this communication.
On the other hand, the packet conversion device on a transmission side of the Patent Document 2 converts a multicast packet into a unicast packet by encapsulation in accordance with a reception request from the packet conversion device on a reception side. Concretely, a client notifies the packet conversion device on a reception side of a desired multicast address, and the packet conversion device on a reception side transmits a reception request of a tree ID corresponding to this multicast address to the packet conversion device on a transmission side. Upon reception of this, the packet conversion device on a transmission side performs the encapsulation with using the request source of this reception request as a destination. Note that, when the packet conversion device on a reception side performs a reception request to the packet conversion device on a transmission side, the packet conversion device on a reception side acquires a correspondence relation among the multicast address, the IP address of the packet conversion device on a transmission side and the tree ID through an address management server, and performs the reception request based on this correspondence relation.